feat(warehouses): backoffice UI for container stack positions

Surfaces the physical stack/slot model in the staff app.

- ZoneLayoutModal: stacks drawn level by level with occupancy colours,
  configured-vs-built-vs-occupied counts, and stack create/delete plus
  block/reserve/free on empty levels
- SlotPicker in the store and move modals, offering only the next
  fillable level of each stack so the form cannot suggest a position
  the API will refuse
- move modal warns when a container is buried, lists the blockers, and
  disables the action instead of firing a 409
- fix: move() now asserts accessibility server-side, matching release —
  both are exits from a stack

apps/edr-freight-api/src/modules/train-scheduling/remainder-placement.service.ts

@@ -333,7 +333,9 @@ export class RemainderPlacementService {
 
     return deferred.map((u) => ({
       containerNumber: u.containerNumber,
-      sealNumber: u.sealNumber ?? undefined,
+      // Deferred units predate the seal requirement; the booking service
+      // normalizes the blank back to null rather than rejecting the re-book.
+      sealNumber: u.sealNumber ?? '',
       vgmTons: Number(u.vgmTons),
       isHazardous: u.isHazardous,
       isReefer: u.isReefer,